Release the GIL while creating a Session. (pantsbuild#13127)

Creating a `Session` accesses the `Graph` to get the number of valid nodes, but when created under the `PySession` wrapper, does so while holding the GIL.

Release the GIL in `PySession`.

[ci skip-build-wheels]
 Conflicts:

[ci skip-jvm-tests]

src/rust/engine/src/externs/interface.rs

@@ -493,14 +493,17 @@ py_class!(class PySession |py| {
           build_id: String,
           should_report_workunits: bool
     ) -> CPyResult<Self> {
-      Self::create_instance(py, Session::new(
-          scheduler_ptr.scheduler(py),
+      // NB: Session creation interacts with the Graph, which must not be accessed while the GIL is
+      // held.
+      let core = scheduler_ptr.scheduler(py).core.clone();
+      let session = py.allow_threads(|| Session::new(
+          core,
           should_record_zipkin_spans,
           should_render_ui,
           build_id,
           should_report_workunits,
-        )
-      )
+      ));
+      Self::create_instance(py, session)
     }
 });
 

src/rust/engine/src/scheduler.rs

@@ -80,7 +80,7 @@ pub struct Session(Arc<InnerSession>);
 
 impl Session {
   pub fn new(
-    scheduler: &Scheduler,
+    core: Arc<Core>,
     should_record_zipkin_spans: bool,
     should_render_ui: bool,
     build_id: String,
@@ -94,7 +94,7 @@ impl Session {
     };
 
     let inner_session = InnerSession {
-      preceding_graph_size: scheduler.core.graph.len(),
+      preceding_graph_size: core.graph.len(),
       roots: Mutex::new(HashMap::new()),
       display,
       should_record_zipkin_spans,